Scorpion venoms have been recognized as a source of peptidyl inhibitors of various types of potassium ion (K) channels. Some of these peptides have been purified to homogeneity and their properties characterized. The most extensively studied of these toxins is charybdotoxin (ChTX). ChTX is a thirty-seven amino acid peptide isolated from venom of the old world scorpion Leiurus quinquetriatus var. hebraeus. Originally described as an inhibitor of the high-conductance, Ca+2-activated K (Maxi-K) channel present in muscle and neuro-endocrine cells, ChTX was later found also to inhibit a number of different medium- and small-conductance Ca+2-activated K-channels, as well as a voltage-dependent K-channel (K(v) 1.3). In each case, channel inhibition occurs with similar potency, in the low nanomolar range. A related toxin, iberiotoxin (IbTX), shares 68% sequence homology with ChTX and selectively blocks the Maxi-K channel. Other peptidyl inhibitors, such as limbatustoxin (LbTX) and kaliotoxin (KTX), have also been shown to possess greater selectivity for the Maxi-K channel. Other peptidyl toxins homologous to ChTX have been identified (e.g., noxiustoxin).
Potassium channels modulate a number of cellular events such as muscle contraction, neuro-endocrine secretion, frequency and duration of action potentials, electrolyte homeostasis, and resting membrane potential. These channels comprise a family of proteins that have been classified according to their biophysical and pharmacological characteristics. Inhibition of K-channels, in their role as modulators of the plasma membrane potential in human T-lymphocytes, has been postulated to play a role in eliciting immunosuppressive responses. In regulating membrane potential, K-channels play a role in the regulation of intracellular Ca+2 homeostasis, which has been found to be important in T-cell activation.
Potassium channel agonists are small polypeptides (31 to 37 amino acids) which form compact structures kept rigid by three disulfide bridges. Use of synthetic analogs with point mutations has determined that single amino acids residues are important for receptor binding and for biological activity of K-channel toxins (Sabatier et al. (1994) Int. J Peptide Protein Res. 43:486–495). Moreover, a drug with high affinity for the receptor could be expected to produce irreversible blockade of synaptic transmission. When labeled with a tracer molecule, such a drug would provide a reliable way of tagging receptors to permit measurement of their number and distribution within cells and tissues. These features would have very valuable consequences for research on excitatory amino acid neurotransmission and for the development of therapeutic agents to treat central nervous system dysfunction in humans and animals. Methods for treating heart and neurological diseases by applying toxins derived from spiders have been described (U.S. Pat. No. 4,925,664).
Arthropod animals, including insects, and certain parasitic worms, use excitatory amino acids as a major chemical neurotransmitter at their neuromuscular junction and in their central nervous system. Because of the damage done by insect pests and the prevalence of parasitic worm infections in animals and humans in many countries, there is a constant need for potent and specific new pesticides and anthelmintic drugs that are non-toxic to humans, pets, and farm animals.
Many arthropods produce a mixture of insecticidal proteins referred to as venom. These toxic substances are synthesized in specialized glandular tissues, which, when directed by a stinging or piercing apparatus, are capable of paralyzing the arthropod's prey. Small, slow moving or stationary arthropods have adapted a strategy to instantaneously paralyze their prey by utilizing neurotoxic components of the venom at very low concentrations. These components or neurotoxins interfere with the function of insect nervous tissues through efficient competition for certain receptor sites. Many of these neurotoxins are polypeptides. These have been divided into different classes based on their host specificity and mode of action (Zlotkin (1991) Phytoparasitica 19:177–182). For example, neurotoxic peptides isolated from numerous species of scorpions have been divided into classes that affect arthropods and classes that affect mammals.
Due to a combination of problems associated with some synthetic insecticides, including toxicity, environmental hazards, and loss of efficacy due to resistance, there exists a continuing need for the development of novel means of invertebrate control, including the development of genetically engineered recombinant baculoviruses which express protein toxins capable of incapacitating the host more rapidly than the baculovirus infection per se.
Scorpion venoms have been identified as possible sources of compounds providing insecticidal properties. Two insect-selective toxins isolated from the venom of the scorpion Leiurus quinquestriatus and affecting sodium conductance have been reported previously (Zlotkin et al. (1985) Arch. Biochem. and Biophysics 240:877–87). One toxin, AaIT, induced fast excitatory contractive paralysis of fly larvae and the other, LqhIT2, induced slow depressant flaccid paralysis suggesting that these two toxins have different chemical and pharmacological properties (Zlotkin et al. (1971) Biochimie (Paris) 53:1073–1078). Thus, other toxins derived from scorpion venom will also have different chemical and pharmacological properties.